# Personal identity: are you a different person every day?

You wake up every morning and your brain and body are not the same as they were the night before. You have some new connections in your brain, while some old connections have been broken. Every cell in your body has undergone changes, some cells have divided, others have died. So are you actually the same person that fell asleep in your bed last night?

For those who believe we have souls the answer is easy - of course you are still the same person because you still have your soul! Even if your body changed completely it's your soul that makes you you. But what about naturalists, people who don't believe in supernatural things like souls, and think that people are just physical systems, very complex ones of course, but still purely physical? If you are a naturalist, are you being inconsistent if you believe that your ten-year-old self from some years ago, you now, and your future self twenty years from now are all one individual, you, despite the fact that your brain and body are very different at these different stages? In fact, your body, for the most part, is composed of completely different atoms than it was when you were ten (I am assuming, perhaps presumptuously, that you are quite a bit older than ten now).

Some think that such naturalists are indeed being inconsistent. A recent short article by Christian philosopher R. Scott Smith criticizes the views of Peter Singer, one of the most influential moral philosophers of our time, who is a naturalist. Singer believes that the reason it's wrong to kill somebody is not because they are a member of the species homo sapiens (that would be "specieist"), but because they have personhood (and members of other species, for example intelligent aliens, can be persons too). He gives several criteria for what constitutes a person, but for our purposes today the important one is this: to be a person one must have the capacity to apprehend oneself as a continuing subject, persisting through time. But, R. Scott Smith claims, this is undermined by Singer's naturalism:

On naturalism, can there literally be an identical person who continues through time and change? There are no essential properties on naturalism. It seems I am just a bundle of physical properties at any given time. That bundle would be identical to another bundle at a different time only if they have all the same properties. But, physical things always are changing. I am changing continuously; some may be relatively minor, e.g., my hair grows, while others may be more significant, such as my growing into adulthood.

What makes all these bundles of properties me at each of these times? The answer seems to be that there is nothing that can do that. My properties keep changing – even the cells in my body and brain. Without something that remains the same, there is no continuing subject, which is a prerequisite for personhood for Singer. Unfortunately, his view entails that there are no persons, which surely is false.

This is not the first time I see objections of this sort to the persistence of personal identity over time, so I wanted to give what I think is a fairly simple response to such line of reasoning. For a physical system to persist through time it is perfectly acceptable for most of its properties to keep changing. For example, a comet flying through space is undergoing constant change as it's losing some of its mass, absorbing radiation, changing its thermodynamic properties as it gets closer to the sun etc.

Yet we don't think it's a completely new celestial object every day. We give it one name, not different names on different days. It's one physical system that evolves in time, and there's nothing bizarre or incoherent about this. On naturalism, we are also physical systems. Just like for the comet, the fact that we evolve in time and our properties don't stay the same doesn't mean we as physical systems don't persist through time.

Of course there is much much more to say about personal identity, it's a large and fascinating subject in philosophy, but I think the above does a decent job sketching how we can persist through time and retain our personal identity even if we are just made of physical stuff, with no supernatural "secret sauce" in the mix.

### 17 Comments - Go to bottom

1. Hey Dmitriy,

The argument implicit throughout your post, seems to go something like this:
1) It's not just persons, all objects (at least the non-atomic kind) undergo radical property modification throughout time.
2) Some objects which display such modification exist (e.g. comets)
3) Therefore radical property modification throughout time cannot entail lack of existence.

The obvious counter to this is to deny premise 2. I think it might be helpful to distinguish between our making reference to an object, and our thinking that such an object actually exists. What you have shown is that we are capable of making reference to objects like comets, people, places, even though such objects have the capacity to undergo radical change. But that's insufficient by itself to establish that such objects actually exist (whatever that means); it could be that such objects are convenient abstractions (fictions) created by our mind to induce order in the phenomenal landscape.

Meaning that in reality there are just atoms/quantum fields (or whatever physicists think there exists) and macroscopic objects are just stable placeholders our brain uses to characterize the world. However, we could have equally, and just as arbitrarily, characterized the world as being composed of different assortments of matter. But obviously that doesn't entail that nothing exists, or that persons don't exist (presumably we have direct access to our own existence). So we need some additional argument to counter Smith's assertions that naturalism fails on account of it failing to give us identity criteria for personhood.

Now there is an implicit assumption in R. Scott Smith's argument (and in most metaphysical circles) that for an object to exist at two different times T1 & T2; some shared property must exist between them. That is, if an object persists through time it must have one or more property(s) that exists in all time periods, which metaphysicists call essential properties. Note that this view (essentialism) is fully compatible with the notion that objects can radically change most of their properties through time, so long as not all of their properties change. Thus, I don't think Smith was trying to imply that an object changing most of its properties means it fails to qualify as an object (as your post seems to argue), but rather that naturalism can't give us any single essential property/condition for personhood.

Now let me tackle Smith's point on that subject. Firstly, we should not underestimate the strength of this challenge; it is surprisingly difficult to give 'stable' necessary and sufficient conditions for personhood or really any kind of object out there. It's not just that objects can lose/modify most of their properties, it's that we are hard pressed to come up with but a single property which we feel that an object (e.g. a comet) must hold in order to be that object. Contemporary philosophers have attempted to identify essential naturalistic properties for personhood/objects, but such attempts have yielded bizarre and non-intuitive results (e.g. ever heard of space-time worms?) and so I won't get into them.

This brings us back to functionalism. The easiest way to uphold naturalism is by introducing a kind of functionalism; this would be an alternative to denying that ordinary objects like comets (but not persons) exist. Here I think Smith implicitly assumes that functional properties are insufficient by their nature to define personhood. But I don't think that follows at all; in fact there are philosophical schools of thought which define the mind as a functional entity. And of course functional mechanisms can persist through time; for example, my body continues to function despite my atomic composition continuously changing.

1. Hey Alex,

you bring up many interesting and complex issues related to personal identity and mereology. But my little blurb doesn't aspire to give a serious account of such issues, it's far less ambitious. It's only meant to give a popular level explanation for why Smith hasn't given a good defense for his statement "Unfortunately, his view entails that there are no persons".

Do you agree that he hasn't? Basically my main point is simply this: he hasn't given a good reason to think that his true statement "physical things always are changing" implies that physical things, including me on naturalism, don't persist through time. He hasn't provided any defeaters to be the prima facia case of normal physical objects, such as comets, persisting while evolving. He hasn't shown any incoherence in such a (standard) view.

2. Hey Dmitriy,

Oh yes I completely agree that his argument, as bare-bones and spartan as it is, is a completely inadequate refutation of Singer's brand of naturalism. That said, I wanted to elaborate on these concepts and also point out that I don't think your rebuttal about the property changing objects necessarily works (I'll elaborate more on that in my next comment). Basically I attempted to fill in some of the gaps/lacunae that Smith's argument had, by elaborating on some of the prominent positions in metaphysical circles.

2. Hey Alex,

To go beyond simply responding to Smith's argument, I wanted to say a couple of things about premise 2:

>2) Some objects which display such modification exist (e.g. comets)

1. Instead of "exist", I would say persist through time.

2. This is indeed a claim I express in the post, without defending it. One reason for not defending it is that I take it to be the standard view. While for some philosophers (though a minority I would guess) arguing for it might be interesting, for the intended reader defending the idea that my apartment existed yesterday would be, I think, like defending that puppies is alive.

3. Another reason for not defending it is because it's not needed to show that there has been no demonstration of Smith's extremely ambitious conclusion that " his view entails that there are no persons". To make that case it would be Smith who would need to make a case that premise 2 is false.

4. But if I had to defend it, how would I? I am not even sure I would try, if by defending it we mean to derive it from some other, more fundamental, assumptions.It's hard to think of assumptions more fundamental than objects persisting through time, to me it is like the assumption that other people are conscious or that the world is more than a minute old. So I would just take it as a properly basic belief or as true by definition of the word "object".

The fact that we call certain structures names like "chair" or "puppy", and certain other structures are left without names is just a subjective feature of our psychology and language. I don't think, for example, that the mereological sum of a chair with a puppy on it is any less of an object in some objective sense than just a normal chair just because we don't have a name for such a structure. Maybe in some other language there is a name for it, like "puppair".

So if concepts like chairs or comets are subjective in this sense then it's really up to us to define them as persisting through time or not, and it seems extremely awkward and contrary to any existing language (I am guessing) and our deepest cognitive structures to define them as being new chairs or comets every nanosecond.

1. Hey Dmitriy,

There is a lot of nuance to tease apart here. I attempted to simplify matters in my previous post in my summary re-hash of your argument. Basically I tried to write-up the argument in the manner I thought you had been perceiving it. But like I said, I think it actually misses the mark somewhat, and much of this is due to the fact that I think Smith is using the word 'exists' in the manner that many (most) contemporary metaphysicists would use the term; which isn't necessarily what you would think.

If I am right and Smith's beliefs about existence do accord with the standard contemporary metaphysical views, then I have a feeling that your views ultimately align more closely with the viewpoint that rejects premise 2. That's because most metaphysicists don't mean by 'exists' what you or I might mean; where I interpret 'exist' to mean that an object is composed of real existing stuff (i.e. its not just a figment of my imagination). Most metaphysicists actually want to go above this more reductive position (that an object is just its mereological sum), and say that objects exist 'above and beyond' their atomic components.

Believe it or not, these metaphysicists spend a lot of time fussing over the conditions over which macroscopic objects come into existence (known as the problem of composition); where they are trying to find unique conditions to justify including 'comet' as an object but not 'puppair', which is basically the complete antithesis to the view you expressed.

I personally think the reductive school of thought (known as the eliminativist position) has great merit. I agree that the starting point should be that objects exist, but once we realize that objects are composed of atomic constituents and that these constituents can completely determine the behavior of the macroscopic bodies, then there is no need keep believing in the 'existence' of the macroscopic objects in the way metaphysicists mean the term. And of course, this is completely separate from whether we should keep talking and referring to such objects; obviously we should for this has great functional purpose.

Now you can see why I didn't talk about this before since it actually seems (at first glance) to go against your argument, which would have excessively complicated things. But hopefully it's clearer now that this is not the case. Of course, most metaphysicists do believe that persons exist (but again, in the stronger sense I described above). It's important to mention that absolutely everyone (even Smith I'm sure) believes that people and all objects exist in the weaker sense I described above. Hence, you can hopefully see that this has just been an equivocation of words. Smith isn't saying the crazy thing you thought he was saying, he wasn't saying that persons don't exist on naturalism in the sense that they are just figments of imaginations, but rather that persons don't exist on naturalism in the stronger sense I implied above. In other words, Smith was afraid that persons are just collections of atoms and their emergent natural properties, which would presumably make him very sad.

Like I said, I feel this whole endeavor is misplaced from the start, or rather I should say that I do agree there is something special about certain kinds of objects like 'comet' instead of 'puppair', but that specialness is purely a product of functionality. In any case, I also feel that Smith is entirely wrong anyway; that actually one can come up with essential 'objective' conditions for personhood that would please the metaphysical crowd that I spoke of above. This is exactly what philosophers of that sort have been doing, and such conditions definitely satisfy naturalism (e.g. space-time worms).

3. Hey Alex,

I think there's some terminological confusion here. First, Smith doesn't even mention the word exist in his article, he is talking about whether persons can persist through time on naturalism.

Second, eliminativism is something different:

Eliminative materialism (or eliminativism) is the radical claim that our ordinary, common-sense understanding of the mind is deeply wrong and that some or all of the mental states posited by common-sense do not actually exist and have no role to play in a mature science of the mind.

I think what you were referring to is instead called mereological nihilism. But I don't subscribe to that position, so I am not departing from most metaphysicists in this regard.

Third, I think it's Smith who is in the minority here. My understanding is that most metaphysicists would agree that comets persist through time. Therefore they don't buy the logic that an object can't persist if its physical properties are constantly changing. So they, like me, don't buy Smith's logic.

4. Hi Dmitriy,

In summary, my view is that there is no fact of the matter. That personal identity is not a real thing at a fundamental level -- there are only pragmatic considerations.

As a pragmatic concern, in most situations, it is not hard to figure out a sense in which someone is the same person they were yesterday. But when we get into thought experiments such as teleportation and mind uploading or even plausible scientific hypotheses such as Everettian many worlds, then it becomes clearer that there isn't a fact of the matter and we shouldn't cling to essentialist notions of personal identity when considering such problems.

So if I upload my mind into a computational substrate, is it still me? I would say so, but only because as a pragmatic matter that self would be competent to play the various roles I play in the world (subject to the handicap of being virtual), such as carrying forward the same concerns I have, relationships with other people, being the bearer of memories, attitudes, skills and foibles, etc. Before the upload, it's up to me whether I deem such a self to be a future me or not. There is no right or wrong final answer, only judgements which rely on reasoning which may or may not be coherent.

Reasons which depend on a soul don't seem good to me, because I don't think souls exist. If on the other hand you want to tie yourself to your specific atoms that seems to run afoul of Ship of Theseus effects over time.

That's not to say that there aren't other approaches which might deem the real you to be the physical one and the virtual one a duplicate. You could coherently identify yourself as some physical locus where the majority of your atoms tend to be over time as atoms come and go. I would reject this not because it's wrong but simply because it is not to my personal taste, being a bit ad hoc. I'm inclined to think that if it's mentally the same as me in every way, then it's me.

If my original physical self persists after the upload, then that would seem to pose a problem, but no more so than the splitting of my identity would on many worlds. The physical and virtual copies would both inherit the personal identity of the pre-upload me, while diverging from then on, so as not to be the same person as each other any more. So personal identity can branch just like a tree or a stream. Potentially two persons could also fuse into one.

1. Yes, coherently identifying a person is difficult. But there are standard metaphysical positions, from three-dimensionalism to four-dimensionalism, which allow us to coherently identify persons (and all objects), and that still accord with naturalism. Though they have weird results (e.g. space-time worms). The alternative is functionalism, where people/objects are defined in functional terms; this also accords well with naturalism and allows us to sidestep concerns about the problem of changing atoms.

5. Hey Dmitriy,

Like I said, this topic is complex and there is a lot of terminology I previously skipped over but which I will now clarify. This will be a two part post.

Part 1)
So eliminativism is a general term in philosophy, just like reductionism. Basically it just refers to any school of thought which thinks that the classical ontology of the field in question is superfluous.

For example, there is eliminative structuralism in philosophy of mathematics, mereological eliminativism for metaphysics, and there is an eliminativist position for philosophy of mind as you mention (eliminative materialism). The latter is the most common use of the term, and so it’s acceptable to simply use the phrase “eliminativism” in that sense. But keep in mind it’s a synecdoche, and when speaking to a philosopher, unless its obvious by context, you need to specify what ontology/position you had in mind. In this context however, eliminativism is identical to the metaphysical nihilist school of thought, which you identified. Both terms are used interchangeably by metaphysicists (see for example: epiphenomenalism and eliminativism by Trenton Merricks).

As for Smith, yes like I said his position definitely goes against contemporary metaphysics and obviously I think his arguments are weak. But like I also mentioned, it’s important to keep in mind that Smith is most likely (I can’t speak for Smith, but I can for standard metaphysics) using the word ‘exists’ in the stronger sense I described above. And that’s the sense that most contemporary metaphysicists mean the term.

“Third, I think it's Smith who is in the minority here. My understanding is that most metaphysicists would agree that comets persist through time. Therefore they don't buy the logic that an object can't persist if its physical properties are constantly changing. So they, like me, don't buy Smith's logic.”

I think there might be some confusion over terminology here as well (partially the fault of us philosophers). Persistence just means to exist at different times; the standard puzzle of persistence is how can an object exist at two different time intervals when it’s in constant flux. Either you believe an object endures through time (it never changes, and is wholly present at each time stage) or that it perdures through time. Here Smith is saying persons neither endure nor perdure through time. I don’t wish to go into too much detail in writing about these schools, but note that when metaphysicists speak of endurance they don’t mean it in the way you might think (i.e. that comets don’t change their physical properties).

Again, it’s important we understand the semantics at play here, because often one’s actual position might be the complete opposite of what they thought because of the language that is being used. It’s definitely a true aphorism of complaint that philosophy often interprets language in ways that are non-standard and don’t accord with common sense, and I find metaphysics to be among the worst offenders in this regard.

So comets, if they existed, have to persist through time; therefore all this (in fact all of metaphysics) is really a puzzle of existence. When we combine that with my previous comment about how objects are thought to exist if they retain some necessary shared property at different times (also known as essentialism) we can now realize that Smith is just saying that persons don’t exist on naturalism in the sense I mentioned; meaning he thinks, on naturalism, there are only person-atoms going through time, not people. I have a feeling that Smith doesn't think that persons are just object-stages, and so he's saying that people don't exist, period (on naturalism).

Hopefully that’s enough background knowledge, and hopefully I’ve made it at least somewhat clear that this is really just the problem of composition for persons.

1. Part 2)

“I think what you were referring to is instead called mereological nihilism. But I don't subscribe to that position, so I am not departing from most metaphysicists in this regard.”

Again it’s important to go back and try to understand what we mean by existence here. A super helpful and short primer would probably be Ernest Sosa’s short article ‘Existential Relativity’ wherein he lays out the main positions, from absolutism to eliminativism, and makes a case for a third way called conceptual relativity which you might find interesting.

You’ll get a clearer sense of what metaphysicists mean when they talk about objects ‘existing’. Like I said, this all boils down to the problem of composition; object relativism is just the position that there is no such objective composition which would entail that comets exist but “puppair” doesn’t. I think this is actually the position which aligns the closest with yours, and with most physicists/scientists. There are two kinds of such relativism:
Absolutism, where basically any possible assortment of atoms is an object, or eliminativism/nihilism where no such assortments exist.

Much of the difference between the two is really semantic; what’s important is that you are (I think) an object relativist but that most metaphysicists (and almost certainly Smith) are not. They think comets objectively exist but that ‘puppair’ doesn’t. Sorry for all the technical talk, it was necessary because I was getting the sense that you thought nihilism/eliminativism meant something else. Like I said, the differences between nihilism and absolutism are mostly really semantic, and so I was using the nihilist position as shorthand for object relativism.

We really should get a sense of just how “weird” much of the standard metaphysical thinking is. Most metaphysicists subscribe to the non-relativist position, which is neither absolutist nor nihilist. It’s the position that objects are not just the sum of their mereological parts, meaning that you can’t just create ‘puppair’ by culturally labeling it. If there was a culture that grew up elsewhere and thought of “puppair” entities as objects then, according to most metaphysicists, they would be wrong.

So hopefully a few things have become clear from all this:

1) This is all intimately tied to the question of existence

2) Smith is almost certainly saying that on naturalism we don’t get objective conditions of the sort I described that would let us say that persons ‘exist’ in the stronger sense. He isn’t saying that there aren’t people-atoms that go through time, or that people are just figments of imaginations (e.g. like for a brain in a vat), or that we can't label such people-atom composites/things that go through time 'people' for convenience.

3) Thus, any talk of how objects obviously exist because we speak of them/recognize them as persisting through time, is based on a semantical misunderstanding of what most metaphysicists mean when they speak of existence.

4) All of this is besides the fact that Smith’s argument is weak (which I obviously agree with).

Best,

Alex

2. In other words,

Smith's position isn't this:
"Therefore they don't buy the logic that an object can't persist if its physical properties are constantly changing."

If Smith is right then people, along with their supernatural souls, would definitely still persist through time, and people's (or at least their bodies if one thought that people endured) physical properties would still be constantly changing. This is just about identifying the essential properties that are necessary to solve the problem of composition for personhood. Smith argues that we need supernatural properties, most metaphysicists say that we don't, a third school (the object relativist school) says that all this talk is misplaced from the very beginning.

6. Hey DM,

I agree that this is mostly a matter of definition. For me personally, physical continuity is important. If someone said: we will make an exact copy of you in the next room, and then kill one of the copies, do you care which one? In fact we will give the surviving you $100000 if you agree that we will kill the version in this room. I will say: no thanks, please kill the version in the next room, if you really have to kill one of us. 1. I guess there's a difference between which answer I think is rationally the most defensible and which answer I might pick out of humility or doubts about my reasoning. I think that * if the clone were so indistinguishable from me so that I should consider myself under SLU as to whether I am the original or the clone * death is instantaneous, such that I'm never suffering or anticipating my death (knowing that I'm the original, post-cloning, and about to die) then strictly speaking I should consider myself to survive and take the$100000.

Whether I would actually agree to that in practice is another matter. I might have doubts about whether those conditions would actually be sufficiently satisfied, or doubts about my own reasoning, which given the stakes would make me wary. I might be 99.9% confident of my position without being willing to bet my life on it.

2. I think the point is that that we're supposed to eliminate all self-location uncertainty (SLU); as Dmitriy writes: "In fact we will give the surviving you \$100000 if you agree that we will kill the version in this room". So it's about killing the being in your room, and you are under no misapprehension as to which being is going to be killed. This makes sense, since the whole point of the thought experiment is to ascertain whether lack of physical continuity is equivalent to death.

3. Hi Alex,

If the cloning is utterly precise, then post-cloning I should be under SLU as to whether I'm the original or the clone, as there should be nothing in my experience to indicate one way or another which I am.

Pre-cloning, when the offer is being made, of course I know I'm the original.

7. Hi Alex,

Thanks for clarifying the terminology, you explained it nicely, so then I would say I am an object relativist of the absolutism variety, because I don't see why some culture would be wrong if they thought "puppair" was an object.

But, importantly, my objection to Smith doesn't presuppose object relativism. Do you agree that Smith's argument is: "on naturalism people are just physical and can't persist because their physical properties are constantly changing"? If that's the argument then it seems to be defeated by finding an example of a physical object that persists even though its properties are constantly changing, for example a comet. That seems to destroy the "because" in his argument.

You suggested that he could deny premise 2, that comets persist. But then he, not I would be in the minority. You said that most metaphysicists would say chairs exist even if many would say puppairs don't. Then do you agree that most would therefore affirm my premise that at least some physical objects persist despite their properties changing?

Moreover, I don't even need to defend premise 2, all I need to do is to say he hasn't argued that it's false, which he would need to do if he wants to argue that properties of a physical human body constantly changing entails that people don't persist on naturalism. If his argument simply takes it for granted that premise 2 is false, then it's a huge assumption that most wouldn't agree with since it implies that on naturalism chairs and comets don't persist.

Of course that still leaves the question: how do people, chairs, and comets persist on naturalism? You sketched some ideas, but again, answering that question is not needed to object to Smith. If space-time worms means we identify the object as it evolves in time by physical continuity then that's my preferred way to identify objects through time.

1. To clarify, I agree with this:

Smith's position isn't this:
"Therefore they don't buy the logic that an object can't persist if its physical properties are constantly changing."

More precisely, I should have said that Smith's argument has as its premise that on naturalism a (physical) object should be presumed not to persist if its physical properties are constantly changing.